This invention relates generally to silicon containing polymers and methods of making, and more particularly, to coating compositions comprising selected substituted polysilazanes and substituted polysiloxazanes which moisture cure at room temperature without requiring a catalyst. The coatings are thermally stable, and in the case of metal substrates, are corrosion-resistant as well. In some cases, the coatings may also be nonwetting. The invention additionally encompasses methods of preparing coated substrates having the aforementioned properties.
A variety of resin systems undergoing moisture cure at ambient temperature conditions have been prepared heretofore. Other than polysilazanes and polysiloxazanes, many employed the hydrolytic sensitivity of isocyanate groups and alkoxy silyl groups to induce room temperature moisture cure. Resin systems utilizing alkoxy silyl groups have been particularly successful. The vast majority of these systems employ tetra-alkoxysilanes (orthosilicates) or oligomers of tetra-alkoxysilanes as moisture curable alkoxy silanes.
One representative example is U.S. Pat. No. 5,413,628 to Savin, which teaches a moisture curable composition comprising an alkyl silicate as a film forming substance, zinc powder, zinc flakes, amorphous silica, and ferrophosphate, wherein the alkyl silicate comprises from about 5% to 20% of tetraethyl orthosilicate.
U.S. Pat. No. 4,810,293 to Sano discloses an anti-corrosive paint composition containing a hydrolyzed condensate of tetraalkoxy silicate, zinc dust, and titanium oxide which undergoes a moisture cure.
U.S. Pat. No. 4,277,284 to Ginsberg et al. discloses a single-package, moisture curable zinc-rich coating composition utilizing zinc, a partially-hydrolyzed organic silicate and an aminosilane.
U.S. Pat. No. 4,571,268 to Frain et al., discloses a single package, moisture curable zinc-filled coating comprising a mixture of partially hydrolyzed ethyl silicate with large amounts of metal particles, such as zinc dust in an organic solvent with an alkali metal nitrate as the catalyst.
U.S. Pat. No. 5,508,360 to Cifuentes et al., discloses a silicon pressure-sensitive adhesive composition curable upon exposure to ambient moisture comprising an organopolysiloxane resin which is appended by alkoxy or oximino radicals.
U.S. Pat. No. 4,525,566 to Homan et al., discloses a moisture-curable silicone composition for adhesive-release coatings comprising an alkoxysilylorganosiloxane, a titanium ester and an aminoorganosiloxane.
U.S. Pat. No. 5,840,794 to Palmer, discloses an RTV sealant composition of a polymer with silicon-containing end groups with at least two silicon-bonded alkoxy groups and a titanium catalyst.
U.S. Pat. No. 5,208,300 to Krahnke et al., discloses silicone pressure sensitive adhesive compositions which are stable in the absence of moisture, but which cure in the presence of moisture comprising an adhesive component bearing alkoxy curing radicals and, optionally, a catalyst to accelerate the reaction of moisture with the alkoxy radicals to form siloxane bonds.
U.S. Pat. No. 5,895,713 to Miyazaki et al., discloses a method for treating an outdoor article, wherein a surface treating agent comprising a medium containing a tetraalkoxysilane or a silane compound having a silanol group obtained by hydrolyzing an oligomer of such a tetraalkoxy silane is used in a moisture curable coating composition.
While polysilazane and polysiloxazane resins have been used to provide coatings with good thermal stability and nonwetting properties, including oxidation and corrosion resistance to metal substrates, most were not moisture curable. Other polysilazanes and polysiloxazane resins provided adherent, protective coatings, but required added catalyst or activator in order to become moisture curable at ambient temperatures.
U.S. Pat. Nos. 5,405,655 and 5,919,572 to Blum et al. disclose certain polysilazane, polysiloxane and polysiloxazane coatings which can be made to undergo cure in the presence of an oxygen donor, such as water with the addition of a ruthenium catalyst. The mechanism of cure does not utilize alkoxy silyl groups as reactive moieties to effect the moisture cure, but rather relies on the activation of Sixe2x80x94H, Nxe2x80x94H, and OH bonds by the metal catalyst to promote crosslinking reactions.
Accordingly, there is a need for silicon-containing polymers which are useful in preparing improved coatings which meet the foregoing criteria, i.e., moisture curable at ambient temperature conditions without requiring added catalyst or activator for rapid curing; wherein the coatings possess desirable properties, namely adherent, protective and heat-stable, while displaying excellent hardness, remain intact even when the substrate is deformed, all with good corrosion resistance over time. Additionally, the silicon polymer-containing coating compositions could be nonwetting.
It has been discovered that novel silazane and siloxazane polymers with selectively substituted alkoxy silyl groups or mixtures of such groups can be employed as resins in coating compositions, wherein the polymers are capable of undergoing rapid moisture cure at room temperature conditions without requiring added catalyst. These moisture curable substituted silazane and siloxazane polymeric coatings not only have protective properties, e.g., corrosion resistance, but provide hard, durable coatings with excellent thermal stability while displaying good adhesion properties to a wide range of substrates.
The silicon-containing polymers are alkoxy silyl substituted silazanes and siloxazanes of the formula:
Zxe2x80x94A
wherein Z is any silazane or siloxazane polymer and A is an alkoxy silyl group of the formula: 
wherein
X is bonded to a silicon atom of Z and is either O or xe2x95x90NR4 wherein R4 is a member selected from the group consisting of hydrogen, lower alkyl, lower alkenyl, aryl, lower alkoxy, silyl, siloxyl and silazanyl;
R1 is alkylene or alkylene substituted with a heteroatom;
R2 is a member selected from the group consisting of
(i) hydrogen,
(ii) hydroxyl,
(iii) C1 to C10 alkyl, either unsubstituted or substituted with hydroxyl, lower alkoxy, lower alkenyl, silyl and xe2x80x94N(R5)2 where R5 is independently selected from the group consisting of hydrogen, lower alkyl and lower alkenyl,
(iv) C1 to C10 alkenyl, either unsubstituted or substituted with hydroxyl, lower alkoxy, silyl and xe2x80x94N(R5)2, and
(v) aryl of 1 or 2 rings, unsubstituted or substituted with hydroxyl, lower alkyl, lower alkenyl, lower alkoxy, silyl and xe2x80x94N(R5)2,
R3 is the same as (iii), (iv) or (v);
n=0, 1 or 2, and the ratio of the total number of OR3 groups of A to Sixe2x80x94N bonds in Z is generally from about 0.05 to about 2.5, preferably from about 0.1 to about 1.0, and most preferably from about 0.1 to about 0.50.
Generally, the novel substituted silazane and siloxazane polymers, Zxe2x80x94A, as disclosed hereinabove, are prepared by the reaction of preformed silazane or siloxazane polymers with certain reagents having reactive amine, ester or hydroxyl functionalities. The novel alkoxy silyl substituted polysilazanes and polysiloxazanes are prepared by heating a mixture of an alkoxy silyl reagent with the chosen polysilazanes or polysiloxazanes in the desired ratio, with the proviso that the ratio of the total number of OR3 groups of A to Sixe2x80x94N bonds in Z, i.e., silazane or siloxazane polymer, prepared by the reaction is from about 0.05 to about 2.5, and preferably from about 0.1 to about 1.0, and most preferably from about 0.1 to about 0.50
The resulting substituted polysilazane or polysiloxazane can then be coated onto a substrate from solution or without the use of a solvent, depending on the physical form of the coating resin. For solid resins and liquid resins which have high viscosity, solvent coating is preferred; for liquid resins having low viscosity, a solventless coating method is preferred.
Coatings prepared by this method are cured in an atmosphere which promotes crosslinking of the polymer by its reaction with moisture. These curing atmospheres include, but are not limited to air and other non-reactive or reactive gaseous environments which contain moisture, such as inert gases like nitrogen and argon, and reactive gases such as ammonia, hydrogen, carbon monoxide, and the like. Rapid cure times are achieved using this method when the applied coatings are exposed to the moisture-containing atmosphere at room temperature.
The cured coatings of the present invention may be used in a wide variety of contexts, insofar as they protect virtually any type of substrate, but particularly metal substrates from oxidative thermal degradation, corrosion, or chemical attack. Additionally, such coatings can also have nonwetting properties, and can be useful in a variety of applications where release characteristics are desirable.
Thus, one principal object of the invention is to provide moisture curable coatings and compositions characterized by thermal stability and corrosion resistance.
It is a further object of the invention to provide coatings of silicon containing polymers with thermal stability properties and, in the case of metal substrates, corrosion resistance.
A further object of the invention is to provide a method of making alkoxy silyl substituted polysilazanes and polysiloxazanes.
It is yet another object of the invention to provide a method for coating substrates with the thermally stable and, in the case of metal substrates, corrosion resistant coatings.
Still another principal object of the invention is to provide coated substrates with protective coatings which are thermally stable and, in the case of metal substrates, corrosion resistant.
In this specification and in the claims which follow, reference will be made to a number of terms and expressions which shall be defined to have the following meanings:
The expression xe2x80x9ccorrosion resistantxe2x80x9d or variation thereof as appearing in the specification and claims refers to coatings on a metal substrate which inhibit the corrosion of the metal substrate when it is exposed to air, heat, or corrosive environments for prolonged time periods.
The term xe2x80x9cnonwettingxe2x80x9d as used herein refers to a substrate surface having a high contact angle with a liquid, typically greater than 90xc2x0. In preferred embodiments, xe2x80x9cnonwettingxe2x80x9d refers to nonwetting characteristics with water.
The term xe2x80x9calkylxe2x80x9d as used herein refers to a branched, unbranched or cyclic saturated hydrocarbon groups having from 1 to 26 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl, and so on. Preferred alkyl groups herein contain 1 to 10, and most preferred from 1 to 8 carbon atoms.
The expression xe2x80x9clower alkylxe2x80x9d as appearing in the specification and claims is intended to embrace alkyl groups ranging generally from 1 to 6 carbon atoms, and more preferably, 1 to 4 carbon atoms. The alkyl groups present on the polymers described herein may be unsubstituted or substituted with one or more functional groups, e.g., halogen, alkoxy, hydroxyl, or the like.
The term xe2x80x9calkylenexe2x80x9d as used herein refers to difunctional saturated branched or unbranched hydrocarbon chains containing from 1 to about 26 carbon atoms, which may be unsubstituted or substituted with various heteroatom containing moieties.
xe2x80x9cLower alkylenexe2x80x9d as appearing in the specification and claims is intended to refer to alkylene linkages containing from 1 to 6 carbon atoms, and includes such representative examples as methylene (xe2x80x94CH2xe2x80x94), ethylene (xe2x80x94CH2xe2x80x94CH2xe2x80x94), propylene(xe2x80x94CH2CH2CH2xe2x80x94), 2-methylpropylene (xe2x80x94CH2 CH(CH3)CH2)xe2x80x94hexylene (xe2x80x94(CH2)6xe2x80x94) and the like, either unsubstituted or xe2x80x9cheteroatomxe2x80x9d group substituted as defined below.
xe2x80x9cHeteroatomxe2x80x9d is intended to mean oxygen, nitrogen, sulfur, phosphorus or other elements which are not carbon or hydrogen.
xe2x80x9cAlkylene substituted with a heteroatomxe2x80x9d as appearing herein is intended to mean heteroatom substitution which may be external to the alkylene hydrocarbon chain or internal to the hydrocarbon chain. 2-hydroxypropyl (xe2x80x94CH2CHOHCH2xe2x80x94), for example, constitutes heteroatom substitution which is external to the alkylene hydrocarbon chain, since carbon bond catenation in the alkylene chain is unbroken by the heteroatom substituent. The alkylene group (xe2x80x94CH2CH2NHCH2xe2x80x94), however, would constitute heteroatom substitution which is internal to the alkylene hydrocarbon chain, since carbon bond catenation is interrupted by the incorporation of Nxe2x80x94C bonds in the alkylene chain.
The term xe2x80x9calkoxyxe2x80x9d is intended to refer to alkyl groups as defined above bound through an ether linkage, typically to a silicon atom.
xe2x80x9cLower alkoxyxe2x80x9d refers to an alkyl-oxygen group containing from 1 to about 6 carbon atoms, and more preferably, from 1 to about 4 carbon atoms.
The term xe2x80x9carylxe2x80x9d as used herein refers to an aromatic species having 1 to 3 ring structures, which may be carbocyclic, heterocyclic, monocyclic or polycyclic, e.g., bicyclic moiety, e.g., phenyl or 1- or 2-naphthyl groups. Optionally, these groups are substituted with 1 to about 4, and more preferably, 1 or 2 lower alkyl, lower alkoxy, hydroxy, amino, and/or nitro substituents.
The term xe2x80x9caminoxe2x80x9d as appearing herein is intended to mean an amino group xe2x80x94NR2 where R is independently selected from hydrogen or an alternative substituent, typically lower alkyl. The term xe2x80x9caminoxe2x80x9d is thus intended to include primary amino xe2x80x94NH2, xe2x80x9calkylaminoxe2x80x9d (i.e., a secondary amino group containing a single alkyl substituent), and xe2x80x9cdialkylaminoxe2x80x9d (i.e., a tertiary amino group containing, for example, two alkyl substituents).
xe2x80x9cHaloxe2x80x9d or xe2x80x9chalogenxe2x80x9d is intended to include fluoro, chloro, bromo or iodo, and often relates to substitution for a hydrogen atom.
xe2x80x9cSilazanesxe2x80x9d as appearing in the specification and claims is intended to include compounds which contain one or more silicon-nitrogen bonds in which the nitrogen atom is bonded to at least two silicon atoms, and may or may not contain cyclic units. The terms xe2x80x9cpolysilazanexe2x80x9d and xe2x80x9csilazane polymerxe2x80x9d are intended to include oligomeric and polymeric silazanes, i.e., compounds which include two or more monomeric silazane units.
xe2x80x9cSiloxazanexe2x80x9d as used herein means compounds which contain one or more silicon-oxygen bonds in which the oxygen atom is bonded to two silicon atoms, plus one or more silicon-nitrogen bonds in which the nitrogen atom is bonded to two or three silicon atoms, and may, or may not contain cyclic units. Thus, the expressions xe2x80x9cpolysiloxazanexe2x80x9d and xe2x80x9csiloxazane polymerxe2x80x9d are intended to include oligomeric and polymeric silazanes, i.e., compounds which include two or more monomeric silazane units, as well as two or more monomeric siloxane units.
The term, xe2x80x9csilyl,xe2x80x9d unless otherwise specified, includes R3Sixe2x80x94, R2HSixe2x80x94, where R is independently selected from the group consisting of hydrogen, alkyl, alkenyl, aryl and silyl; H3Sixe2x80x94, siloxyl, siloxazyl, and silazyl, and further, includes repeating silyl units, or xe2x80x9cpolysilylxe2x80x9d species.
These inventors discovered that selectively substituted silazane and siloxazane polymers comprising alkoxy silyl substituents can be employed as coating resins which undergo rapid moisture cure at room temperature without the need for added catalyst. The selectively substituted silazane and siloxazane polymers of this invention are those wherein the ratio of the total number of OR3 groups of A to Sixe2x80x94N bonds in Z is generally from about 0.05 to about 2.5, preferably from about 0.1 to about 1.0, and most preferably from about 0.1 to about 0.50.
The novel substituted silazane and siloxazane polymers are prepared by the reaction of a preformed silazane or siloxazane polymers with reagents having reactive amine or hydroxyl functionality of the structural formula: 
wherein
Y is either OH, OCOR6, or NHR6 where R6 is a member selected from the group consisting of hydrogen, C1 to C8 alkyl, C1 to C8 alkenyl, lower alkoxy, aryl and silyl;
R1 is alkylene or alkylene substituted with a heteroatom;
R2 is selected from the group consisting of
(i) hydrogen,
(ii) hydroxyl,
(iii) C1 to C10 alkyl, either unsubstituted or substituted with hydroxyl, lower alkoxy, lower alkenyl, silyl and xe2x80x94NR5 where R5 is independently selected from the group consisting of hydrogen, lower alkyl and lower alkenyl,
(iv) C1 to C10 alkenyl, either unsubstituted or substituted with hydroxyl, lower alkoxy, silyl and xe2x80x94NR5, and
(v) aryl of 1 or 2 rings, unsubstituted or substituted with hydroxyl, lower alkyl, lower alkenyl, lower alkoxy, silyl and xe2x80x94NR5,
R3 is the same as (iii), (iv) or (v);
n=0, 1 or 2.
The alkoxy silyl substituted polysilazanes and polysiloxazanes of the present invention resulting from the reaction of the alkoxy silyl reagents described above with polysilazanes, polysiloxazanes,or mixtures thereof comprise: 
wherein Z is the preformed silazane or siloxazane polymer and X is bonded to a silicon atom of Z, and the values for X, R1, R2, R3 and n of the alkoxy silyl group are the same as disclosed hereinabove.
The base ingredients in the inventive compositions include preformed polysilazanes and polysiloxazanes. Any polysilazane known in the art or commercially available having repeating units of Sixe2x80x94N linkages, including mixtures of polysilazanes can be used to prepare the novel alkoxy silyl substituted compositions of the invention. Similarly, any known or commercially available polysiloxazane and mixtures thereof can be used to prepare the compositions. The alkoxy silyl substituted coating compositions may also be prepared with heterogeneous mixtures containing both preformed silazane and siloxazane polymers.
As stated above, xe2x80x9cpolysilazanexe2x80x9d as appearing herein is intended to include any oligomeric or polymeric composition. In addition, the term xe2x80x9cpolysilazanexe2x80x9d denotes a compound which includes one or more Sixe2x80x94N units in which the nitrogen atoms are bonded to at least two silicon atoms. By xe2x80x9coligomerxe2x80x9d is also intended to mean a molecule or chemical compound comprising several repeat units, generally from about 2 to 10 repeat units. xe2x80x9cPolymerxe2x80x9d, as used herein, means a molecule or compound which comprises a large number of repeat units, generally greater than about 10 repeat units.
The oligomeric or polymeric silazanes of this invention may be amorphous or crystalline in nature. Such compositions may be liquid or solids which are cyclic, linear, or cyclolinear in nature. Preferably, the polysilazane has at least one silyl amine group, and more preferably, at least two terminal and primary silyl amine groups as part of its composition, although the presence of at least two Sixe2x80x94N bonds within the molecule suffices to provide the reactivity essential to the preparation of the novel compositions of the invention.
Representative examples of various polysilazanes that can be used include, but are not limited to, polymers described in U.S. Pat. No. 4,482,669 entitled xe2x80x9cPreceramic Organosilazane Polymersxe2x80x9d issued to Seyferth et al; U.S. Pat. No. 4,774,312 entitled xe2x80x9cPolydisilacyclobuta-silazanesxe2x80x9d issued to Burn: U.S. Pat. No. 4,689,252 entitled xe2x80x9cPolysilazane Compositions which can Crosslink in the Presence of a Metal Compound Catalyzing a Hydrosilylation Reactionxe2x80x9d issued to Laine et al; U.S. Pat. No. 4,612,383 entitled xe2x80x9cMethod of Producing Polysilazanesxe2x80x9d issued to King et al.
Also included within the definition of xe2x80x9cpolysilazanesxe2x80x9d or silazane polymers are the xe2x80x9cpolyureasilazanesxe2x80x9d, which includes the poly(thio)ureasilazanes, such as disclosed by U.S. Pat. No. 5,155,181 entitled xe2x80x9c(Thio)amide-Modified Silazane Polymer Composition Containing a Free Radical Generatorxe2x80x9d by Schwark; U.S. Pat. No. 4,929,704 for xe2x80x9cIsocyanate- and Isothiocyanate-Modified Polysilazane Ceramic Precursorsxe2x80x9d also to Schwark, and U.S. Pat. No. 5,021,533 for xe2x80x9cCrosslinkable Poly(thio)urea-silazane Composition Containing a Free Radical Generatorxe2x80x9d, also to Schwark. The entire disclosures of these U.S. Patents and the references contained therein are hereby incorporated herein by reference.
In the alternative, and preferably, novel polysilazanes may be prepared according to the methods set forth in U.S. Pat. No. 6,329,487, dated Dec. 11, 2001, the contents of which are incorporated herein-by-reference.
Briefly, the novel polysilazanes prepared are characterized by repeating units of silicon-nitrogen linkages and comprising a reduced amount of Sixe2x80x94H bonds relative to the quantity of Sixe2x80x94H bonds found in the halosilanes used to prepared the novel polysilazanes. The term halosilanes is meant to include organohalosilanes, as well inorganic species.
The preferred novel polysilazanes are prepared by reacting a halosilane, that must have at least one Sixe2x80x94H bond, with anhydrous liquid ammonia, where the halosilane has the general formula: 
where m is 0 to 2, n is 1 to 3, provided the sum of m+n is from 1 to 3.
While the halosilanes of formula (A) are especially preferred, other representative halosilanes useful in the preparation of the coating compositions of the present invention includes those compounds represented by the structural formula: 
where m is 1, 2 or 3.
In each of the above halosilanes (A and B) R may be identical or different, i.e., independently selected, and can be a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group and a substituted or unsubstituted aryl group; X is a halogen atom selected from bromine, chlorine or iodine. Preferably, when m is 2, each R is the same, as those silanes are more readily available and easier to make. When R is methyl or ethyl, X is preferably chlorine, m is preferably 1 and n is preferably 1.
Representative examples of suitable halosilanes and organohalosilanes include, but are not limited to, dichlorosilane, methyldichlorosilane, dimethylchlorosilane, dimethyldichlorosilane, diethylchlorosilane, ethyl dichlorosilane, ethyl dibromosilane, ethyldiiodosilane, ethyl difluorosilane, dichloromonofluorosilane, propyldibromosilane, isopropyldichlorosilane, butyldiiodosilane, n-propyldichlorosilane, dipropylchlorosilane, trichlorosilane, n-butyldichlorosilane, isobutyldichlorosilane, isoamyldichlorosilane, benzyldichlorosilane, naphthyldichlorosilane, propenyldichlorosilane, phenyldichlorosilane, diphenylchlorosilane, methylethylchlorosilane, vinylmethylchlorosilane, phenylmethylchloro-silane, dibenzylchlorosilane, p-chlorophenylsilicon dichloride, n-hexyldichlorosilane, cyclohexyldichlorosilane, dicyclohexylchlorosilane, diisobutylchlorosilane, paratolyldichlorosilane, diparatolyl-chlorosilane, parastyryldichlorosilane, ethynyldichlorosilane, and mixtures thereof.
The preferred organohalosilanes are methyldichlorosilane and dimethyldichlorosilane, as they are commercially available and work well.
Mixtures of halosilanes are also useful in preparing the polysilazane reactant because the organic groups attached to silicon can often impart a variety of chemical and/or physical characteristics of use to the polymer. For instance, organic residues of high hydrocarbon content can assist in hydrocarbon solvent solubility, as well as confer a greater degree of hydrolytic stability to the polysilazane.
To confer some of the desirable characteristics certain organic residues can impart to the polysilazane reactant, the novel halosilanes of general formula (A) can be mixed with up to about 90 wt % of a halosilane that does not contain an Sixe2x80x94H bond, such as species falling within the genus of general formula (B) before the preparation of the polysilazane via ammonolysis in anhydrous liquid ammonia. Preferably, the halosilane of the above general formula (A) is mixed with the second halosilane that does not contain an Sixe2x80x94H bond prior to ammonolysis, for the sake of simplicity.
The novel polysilazane compounds represented by structural formula (A) in the preferred embodiment described above are prepared by ammonolysis, the method comprising the following steps:
introducing at least one halosilane having at least one Si-hydrogen bond into anhydrous liquid ammonia wherein any formed ammonium halide salt is solubilized and provides an acidic environment therein; and
maintaining the formed silazanes in the acidic environment for further polymerization thereby reducing the number of Sixe2x80x94H bonds that were present initially in the starting halosilane.
No metal catalyst or other added catalytic agent is required to produce the preferred silazane polymers described above.
The selected halosilane or mixtures thereof are introduced directly into and reacted with anhydrous liquid ammonia. Normally, during ammonolysis, on a strictly stoichiometric basis, two molecules of ammonia are needed for each halogen atom substituted on a halosilane. One ammonia molecule replaces the halogen atom, while the second molecule of ammonia forms an ammonium halide salt. In this regard, it has been found to be advantageous to introduce the halosilanes into a closable reaction vessel which is already charged with an excess of anhydrous liquid ammonia, preferably, at least twice the amount of ammonia as Sixe2x80x94X bonds present. More preferably, it is desirable to use at least five times the amount of ammonia as Sixe2x80x94X bonds.
The anhydrous liquid ammonia is maintained at a sufficient temperature and/or pressure to remain in a liquefied state and, preferably, between about xe2x88x9233xc2x0 C. to about 130xc2x0 C. As a result, the anhydrous ammonia, in a liquefied state, acts as a reactive solvent that not only participates as a nucleophile in the nucleophilic attack on the halosilane, but also solubilizes and retains a substantial amount of ammonium halide salt produced during ammonolysis.
While not wishing to be bound by any particular theory of operation, it is believed that by retaining the solubilized and ionized ammonium halide in the liquid ammonia solution, the ionized salt acts as an effective catalyst in the polymerization process to produce novel polysilazanes which are depleted in Sixe2x80x94H content from their original composition without the use of added metallic or other added catalysts.
It has been observed that, initially, the reaction proceeds in a homogeneous phase wherein the generated ammonium halide salt is solubilized and ionized in the anhydrous liquid ammonia solution of silane ammonolysis product, thereby reducing precipitation of ammonium halide salt and allowing the solubilized salt to act in catalytic fashion to polymerize the halosilane ammonolysis products. This process is accompanied by a reduction in Sixe2x80x94H bonds in the product relative to the number present in the halosilane reactants and ammonolysis products. Once formed, the polysilazane product having reduced Sixe2x80x94H content is then found to spontaneously separate from the liquid ammonia phase. In this new phase layer it is essentially free from ammonium chloride salt contamination.
As such, solubilization of ammonium chloride avoids contamination of the ammonolysis products with precipitated salt. The process described above also eliminates the need for introducing an inert solvent to reduce the viscosity of the reaction mixture, as described in prior art. A distinct disadvantage of adding such inert solvent, is the problem of dealing with the large evolution of heat which accompanies the crystallization of the ammonium chloride from the reaction mixture. This problem is circumvented in the process described above, since the ammonium chloride remains in solution in the anhydrous liquid ammonia.
The mechanism for further polymerization of silazanes formed during the ammonolysis process where a reduction in the number of Sixe2x80x94H bonds present in the initial ammonolysis products occurs is not completely understood. Unexpectedly, further polymerization through ammonolysis can be effected without active silicon-halogen ammonolysis sites on a starting compound which has at least one Sixe2x80x94H bond. It is believed that heterolytic cleavage of the Sixe2x80x94H bond provides a route for further ammonolysis until all active Sixe2x80x94H sites are cleaved and reacted and/or the preferred viscosity is achieved.
The novel polysilazanes preferred for use in the present invention are characterized by having a decreased number of Sixe2x80x94H bonds relative to the amount of Sixe2x80x94H bonds contained in the starting compounds. The reduction in Sixe2x80x94H bonds can range from about 10% to about 90% relative to the number of Sixe2x80x94H bonds contained in the starting compounds. Additionally, it has been found there is a proportional increase in the Sixe2x80x94N linkages which is essentially proportional to the reduction in Sixe2x80x94H bonds. The novel polysilazanes of structural formula (A) are believed to comprise several different structures including linear, branched, ladder, and fused ring morphologies, although it is believed that these novel polysilazanes have fewer isolated ring structures than prior polysilazanes.
Representative examples of a polysilazane having fused six and eight membered rings are shown in structures (1) and (2) below. These structures are merely representative of the novel polysilazanes prepared with the halosilanes of structural formula (A) employing the novel methods disclosed in U.S. Pat. No. 6,329,487, wherein R is independently selected from the group consisting of hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted alkenyl or substituted or unsubstituted aryl group, and n is 1 or greater. 
While not wishing to be bound by theory, it is believed that the initial reaction leading to the formation of these novel ammonolysis products may be represented generally by the following scheme showing a possible mechanistic route using a Sixe2x80x94H bond containing starting compound, such as methyldichlorosilane: 
During the initial ammonolysis, the silicon-chlorine bonds undergo ammonolysis, generating a diaminosilane which is further converted into a linear molecule containing several Sixe2x80x94N repeat units. The linear structure is stabilized in the anhydrous liquid ammonia containing an ionized ammonium halide salt dissolved therein. This ionized and dissolved ammonium halide salt then acts as an acid catalyst which catalyzes a loss of a Sixe2x80x94H bond to generate a new silicon-chlorine bond on the straight chain of the polymer. The newly generated chlorosilane bond may then undergo further ammonolysis. This reaction will proceed until virtually all chlorosilicon bonds undergo ammonolysis.
The polymerization can be performed in the same reactor and at the same conditions used for ammonolysis and without isolation of the initial silazane ammonolysis product. Thus, ammonolysis and polymerization may all be occurring at the same time. Two easily-separated phases are formed and the novel polysilazane can be easily isolated in a relatively pure state by filtering and washing with either additional liquid ammonia, or a simple hydrocarbon solvent, such as hexane or heptane.
The foregoing novel polysilazanes used as starting materials in the preparation of the moisture curable coating compositions of this invention are also commercially available under the Kion(trademark) from the Kion Corporation, Columbus, Ohio.
In addition to the polysilazane reactant (Z), any polysiloxazane or mixture of polysiloxazanes with one another, or with polysilazanes known in the art or commercially available can be used to prepare the novel alkoxy silyl substituted compositions of this invention. In addition to the definition given above, the term xe2x80x9cpolysiloxazanexe2x80x9d is also intended to mean any oligomeric or polymeric composition comprising a plurality of Sixe2x80x94N repeat units, as well as a plurality of Sixe2x80x94O repeat units. The oligomeric and polymeric siloxazane reactants of this invention may be amorphous or crystalline in nature. Such compositions may be liquid or solid, cyclic, linear, or cyclolinear in nature. The polysiloxazanes preferably have at least one silyl amine group, and more preferably, at least two terminal and primary silyl amine groups as part of its composition, although the presence of at least two Sixe2x80x94N bonds within the molecule suffices to provide the reactivity essential to the preparation of the novel compositions of this invention.
Representative examples of various polysiloxazanes that can be used include, but are not limited to, polymers described in U.S. Pat. Nos. 5,405,655 and 5,919,572 to Blum et al.
In addition to the polysilazanes, polysiloxazanes and mixtures thereof discussed hereinabove for preparing the coating compositions of this invention, other silicon-containing polymers (Z) may be employed in making the alkoxy silyl substituted silicon-containing polymers. Representative examples are the novel block copolymers disclosed in copending U.S. patent application Ser. No. 09/793,224, filed Feb. 26, 2001, now U.S. Pat. No. 6,534,104 the contents of which are incorporated herein-by-reference. This application teaches the preparation of block copolymers of silazanes and siloxanes by the reaction of preformed polysilazanes with silyl hydroxy or silyl amine terminated polysiloxanes.
The polysilazane blocks described in the above copending application may be oligomeric or polymeric in nature, cyclic, linear, or cyclolinear in structure, and either liquid or solid in form. Preferably, each polysilazane contains at least four repeating units, and more preferably, from about six to greater than about 20 repeating units. The polysilazane blocks may contain at least two silyl amine groups through which they can polymerize with functional groups, such as hydroxyl groups of the polysiloxane blocks. Notwithstanding, the presence of at least one Sixe2x80x94N bond within the polysilazane would suffice to promote such reactivity when a reactive, nucleophilic group is used to promote the copolymerization of the polysilazane and polysiloxane blocks used in the preparation of the novel block copolymers. The reaction of an Sixe2x80x94OH group with an Sixe2x80x94NRH group (wherein R=hydrogen, alkyl, substituted alkyl, cycloalkyl, etc . . . ) results in the formation of an Sixe2x80x94Oxe2x80x94Si bond scheme with the evolution of ammonia or an amine. The reaction of an Sixe2x80x94OH group with a Sixe2x80x94NRxe2x80x94Si group results in the formation of an Sixe2x80x94Oxe2x80x94Si group and an Sixe2x80x94NRH group. This Sixe2x80x94NRH group can subsequently react with an additional Sixe2x80x94OH group to form an Sixe2x80x94Oxe2x80x94Si group and a molecule of ammonia or amine, as described above. Thus, in a preferred embodiment of the invention, polysilazanes are reacted with xe2x80x94OH functional polysiloxanes to prepare block copolymers in which the blocks of polysilazane and polysiloxane are joined by Sixe2x80x94Oxe2x80x94Si bond linkages.
Although the above description illustrates the reactivity of an Sixe2x80x94OH group with a polysilazane, a variety of other nucleophilic groups may be used to prepare the block copolymers. Thus, the polysiloxane used in the practice of the invention may comprise such nucleophilic groups as xe2x80x94OH, xe2x80x94NH2, xe2x80x94NRH, xe2x80x94CO2H, xe2x80x94SH, and others to effect the desired covalent linkages between the polysilazane blocks and the polysiloxane blocks of the novel block copolymers. While this group is representative of the types of nucleophilic moieties which can react with Sixe2x80x94N bonds in the polysilazanes used in the practice of the invention, it is not limiting. Indeed, any nucleophilic group which can cleave the Sixe2x80x94N bond can be used to prepare these novel compositions. The nucleophilic group may be bonded directly to silicon, or may be present at the terminus of a xe2x80x9cspacerxe2x80x9d group which is bonded to silicon. Accordingly, a polysiloxane which is terminated with an aminopropylsilyl group is also suitable for the practice of the invention:
H2Nxe2x80x94(CH2)3xe2x80x94Si(CH3)2[xe2x80x94Oxe2x80x94Si(CH3)2xe2x80x94]nxe2x80x94O""Si(CH3)2xe2x80x94(CH2)3xe2x80x94NH2
The polysilazane blocks are copolymerized with any polysiloxane or mixtures of different polysiloxanes that are commercially available, or can be prepared by methods well-known in the art. The polysiloxane may contain a linear, branched, or cross-linked polymeric system of alternating silicon and oxygen atoms having the general formulas: 
where R may be the same or different and includes, but is not limited to hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, and a substituted or unsubstituted aryl group and the like.
Preferably, the polysiloxane contains at least two reactive nucleophilic functional groups including hydroxyl groups, amine groups, acid groups, and thiol groups. More preferably, a polysiloxane contains hydroxyl groups. It is believed that a polysiloxane polymer having at least two, and preferably more nucleophilic substituents acting as reactive groups attack the Sixe2x80x94NH2 or Sixe2x80x94NHxe2x80x94Si bonds of the polysilazane as described above, resulting in the breaking of these bonds and the formation of new bonds. The polysiloxane should contain at least 4 repeating units; preferably the polysiloxane contains about 6 to greater than about 20 repeating units to maximize the advantageous properties of each segment in the block copolymer.
The polysilazane/polysiloxane block copolymer can be prepared by polymerizing the polysilazane with the polysiloxane. The weight ratio of the polysilazane to the polysiloxane should be about 15:85 to about 85:15 as at ratios above or below these full copolymerization of the blocks is often not effected; the preferred polysilazane/polysiloxane weight ratio is about 20:80 to about 80:20. At least about 90 wt % of the copolymer should be made from polysilazane and polysiloxane blocks that are each at least 10 wt % of the copolymer. When xe2x80x94OH or xe2x80x94NH2 functional polysiloxanes are used to prepare the novel block copolymers, no catalyst is required. Also, no solvent is required during polymerization, but a solvent can be added to reduce the viscosity, if desired.
While copolymerization can be performed in preparation of the blocks at temperatures approximating room temperature, or even below, more economically attractive rates of reaction can be achieved by heating reaction mixtures to at least 45xc2x0 C., and more preferably, from about 65xc2x0 C. to about 100xc2x0 C. The reaction typically requires from about one to about two hours. The reaction is complete when off gassing ceases. While the block copolymer will flow freely without added solvent, the viscosity can be reduced, if desired, by the addition of an aromatic hydrocarbon solvent such as, toluene, xylene, an aliphatic hydrocarbon solvent such as heptane, decane, or dodecane, an ether solvent such as tetrahydrofuran or anisole, an ester solvent, such as hexyl acetate or butyl propionate, or a ketone solvent such as acetone, methylethylketone, and the like.
As disclosed hereinabove, the novel moisture curable polysilazanes and polysiloxazanes are prepared by reacting a preformed polysilazane or polysiloxazane with reagents having reactive amine or hydroxyl functionality. Alkoxy silyl reagents useful in the practice of this invention have the structural formula: 
wherein Y is either OH, OCOR6 or NHR6, and the values for R1 to R3, R6 and n are the same as those disclosed heretofore.
Representative alkoxy silyl reagents useful for the practice of this invention include, but are not limited to acetoxymethyltriethoxysilane, acetoxymethyltrimethoxysilane, acetoxypropyltrimethoxysilane, N-(3-acryloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane, (3-acryloxypropyl)dimethylmethoxysilane, (3-acryloxypropyl)methyldimethoxysilane, (3-acryloxypropyl)trimethoxysilane, 3-(N-allylamino)propyltrimethoxysilane, 4-aminobutyltriethoxysilane, N-(2-aminoethyl)-3-aminoisobutylmethyldimethoxysilane, (aminoethylaminomethyl)phenethyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(6-aminohexyl)aminopropyltrimethoxysilane, 3-(m-aminophenoxy)propyltrimethoxysilane, m-aminophenyltrimethoxysilane, p-aminophenyltrimethoxysilane, 3-(3-aminopropoxy)-3,3-dimethyl-1-propenyl-trimethoxysilane, 3-aminopropyldimethylethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-aminopropyltriethoxy-silane, 3-aminopropyltrimethoxysilane, 3-aminopropyltris(methoxyethoxyethoxy)silane, benzoyloxypropyltrimethoxysilane, bis-(2-hydroxyethyl)-3-aminopropyltriethoxysilane, hydroxymethyltriethoxysilane, (methacryloxymethyl)dimethylethoxysilane, methacryloxymethyltriethoxysilane, methacryloxymethyltrimethoxysilane, methacryloxypropyldimethylethoxysilane, methacryloxypropyltriethoxysilane, methacryloxypropyltris(methoxyethoxy)silane, N-methylaminopropylmethyldimethoxysilane, N-methylaminopropyltrimethoxysilane, N-phenylaminopropyltrimethoxysilane, (3-trimethoxysilylpropyl)diethylenetriamine.
Acetoxymethyl-triethoxysilane, acetoxymethyltrimethoxysilane, acetoxy-propyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltri-methoxysilane, 3-(3-aminopropoxy)-3,3-dimethyl-1-propenyl-trimethoxysilane, 3-aminopropyltriethoxysilane, 3-amino-propyltrimethoxysilane, and (3-trimethoxysilylpropyl)diethylenetriamine are preferred.
Most preferred are member selected from the group consisting of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, and (3-trimethoxysilylpropyl)diethylenetriamine.
Specific representative alkoxy silyl substituted polymers of the invention include the following:
wherein Z contains only hydrogen and methyl groups bonded to silicon.
The novel alkoxy silyl substituted polysilazanes and polysiloxazanes of the present invention are prepared by simply heating the desired polysilazane or polysiloxane or mixtures of polysilazanes and polysiloxazanes in the presence of the alkoxy silyl reagent. An amount of alkoxy silyl reagent is chosen such that the final composition contains a ratio wherein the total number of OR3 groups of A to Sixe2x80x94N bonds in Z is generally from about 0.05 to about 2.5, preferably from about 0.1 to about 1.0, and most preferably from about 0.1 to about 0.50.
The reaction is typically performed without solvent, since the preferred alkoxy silyl reagents, polysilazanes, and polysiloxazanes are typically liquids. However, the reaction may optionally be performed in solvent. Suitable solvents include hydrocarbon solvents such as toluene, hexane, or heptane, ether solvents such as tetrahydrofuran or dimethoxyethane, or dry ketone solvents such as acetone, methyl ethyl ketone, and the like.
The reaction is generally conducted in a range from about 25xc2x0 to about 150xc2x0 C., and more specifically, from about 50xc2x0 and 120xc2x0 C., and most preferably, from about 80xc2x0 to about 110xc2x0 C. Times for the reactions to go to completion generally range from about 10 minutes to about 2 hours.
When solvent is omitted, the reaction product may be used directly upon cooling with no further work-up. When a solvent is employed, the solvent may be removed by conventional techniques, such as distillation. Alternatively, the solution of the reaction product in the solvent may be used directly to coat a substrate.
Coated substrates according to this invention are prepared by coating the alkoxy silyl substituted polysilazane or polysiloxazane composition directly onto the substrate of interest, provided the polymer is a low viscosity liquid. When the polymer is a solid or a high viscosity liquid, the alkoxy silyl substituted polysilazane or polysiloxazane can be dissolved in a compatible organic solvent to prepare a coating solution.
Preferred solvents for preparing the coating solutions are such that they do not react with the polymer (e.g., alcohol or amine solvents should generally be avoided when preparing coatings of polysilazanes and polysiloxazanes when solvent reactivity is desired), and are sufficiently volatile to facilitate drying the coatings. Also, it is desirable to avoid use of hazardous solvents, such as benzene, trichloroethylene, and the like. Examples of particularly preferred solvents include toluene, tetrahydrofuran and hexane. Generally, coating solutions are formulated with from 0.1 wt % to 99 wt % polymer, and more specifically from 0.1 wt % to about 20 wt % polymer, and most preferably from 0.1 wt % to about 10 wt % polymer.
It may also be desirable to incorporate organic or inorganic powders into the coating compositions. This may be done for a number of reasons, the most obvious of which are to increase the viscosity of the coating solutions to enable preparation of a paste, or a relatively thick solution so that it may be xe2x80x9cpaintedxe2x80x9d onto a substrate, or to impart specific performance characteristics to the coating. For example, metal powders, such as copper, iron, zirconium, titanium, tungsten, molybdenum, zinc, and aluminum may be admixed with the polymeric solution prior to coating. Such a technique is useful, for example, to provide an anti-corrosion barrier on the surface of a metallic substrate. Incorporation of metal powder into the coating solution is also useful to prepare a harder coating, regardless of the substrate (in which case preferred metal powders include zirconium, titanium, tungsten, molybdenum and hafnium powders).
It may also be desirable to incorporate ceramic powders and glasses, such as silicon carbide, silicon nitride, boron carbide, titanium carbide, titanium nitride, tungsten carbide, molybdenum oxide, silica and aluminum oxide, typically for the purpose of creating a harder coating, but also useful for providing a nonconductive surface on a conductive substrate, for providing corrosion-resistant coatings, impact-resistant coatings, and coatings having a mismatched thermal expansion coefficient, i.e., relative to the substrate surface. Inclusion of silica, boron nitride, aluminum nitride or beryllium oxide powders in the coating solution is desirable in electronics applications, insofar as these materials are good dielectrics. Carbon powder (including pyrrolytic carbon powder and graphite powder) and organic powders such as Teflon(copyright), polysiloxane, poly-carbonate, or polyamide powders may also be used to thicken the coating solutions. In addition, various mineral powders may be employed, such as clays, which can act as thixotropes.
The invention contemplates coating a wide variety of substrates employing the polymeric compositions and methods disclosed herein. They include substrates comprised of metallic, ceramic, glass and organic materials, such as polyesters, nylons, acrylates, and so on. Specific representative examples of metal substrates include steel, aluminum, copper and alloys thereof. Examples of ceramics include silicon nitride, silicon carbide, silica, alumina, zirconia, and the like. It will be appreciated by those skilled in the art that the foregoing lists are merely illustrative of various materials which may be coated using the presently disclosed compositions and methods, and are not in any way limiting of the different substrates with which the present invention is useful.
It will also be understood, the present methods are also useful in coating substrates having different shapes, e.g., substrates having flat, planar surfaces, molded articles having curved surfaces, fibers, fabrics, and the like.
Coatings may be applied by dipping the substrate in the aforementioned coating compositions. Alternatively, the coatings may be applied by painting, wiping, spraying or spin-coating techniques. These procedures will typically provide pure polymer coatings having thicknesses of up to about 75 microns per coat for the cured polymers, but may provide coatings on the order of 1 micron or even thinner, if desired. If a thicker coating is desired, multiple coating layers may be provided. The layers may be comprised of the same or different polymeric materials, and may be applied from coating compositions which may or may not contain solvent.
Coatings may be cured at temperatures ranging from room temperature to about 50xc2x0 C. Preferably, curing is performed at temperatures not exceeding about 100xc2x0 C.
Coatings prepared by methods of the invention are cured in an atmosphere which promotes crosslinking of the polymer by its reaction with moisture. These curing atmospheres include, but are not limited to, air and other non-reactive or reactive gaseous environments which contain moisture, inert gases like nitrogen and argon, and reactive gases such as ammonia, hydrogen, carbon monoxide, and so on. Rapid cure times are achieved using this method when the applied coatings are exposed to the moisture-containing atmosphere at room temperature.
The coatings so provided may be used in a wide variety of contexts, insofar as they protect virtually any type of substrate from oxidative thermal degradation, corrosion, or chemical attack. The coatings may also be used to strengthen relatively flaw sensitive brittle substrates such as glass and, as noted earlier herein, can be useful for providing a nonwetting surface. The coatings may additionally be useful to provide electrical insulation and/or bonding or compatibility interfaces between different types of materials.